Tuna Pasta Bake

This recipe is from the back of Campbell's Cream of Celery condensed soup (with a couple of my own modifications), and I've made it many times.

1 tbsp oil
1 onion, finely chopped
1/2 green capsicum, diced
1 x 410g can Campbell's Condensed Cream of Celery soup
425g can tuna, drained
250g dried large shell pasta, cooked and drained
1 1/2 cups grated tasty cheese

1. Preheat oven 200 deg Celsius. Lightly grease an 8-cup (2L) capacity ovenproof dish.
2. Heat oil in a large frying pan over medium heat. Add onion and cook, stirring occasionally, for 5 minutes or until onion is soft.
3. In a large bowl mix onion, capsicum, soup, tuna, pasta and most of the cheese until combined then place into prepared ovenproof dish. Sprinkle with remaining cheese and bake for 20 minutes or until golden brown.